System and method for controlling a solar panel output

ABSTRACT

A control system includes a control module and one or more input sources. The control module is coupled to an output of the solar module in order to operate the solar panel so that an output of the solar panel is at a maximum power level. The control module is able to selectively decrease a current level of the solar panel&#39;s output in response to a condition that is indicative of a temperature of the solar panel while maintaining the power output of the solar panel at or within a designated percentage of the maximum level. The input source is coupled to the control module to provide an input that is indicative of the temperature.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.12/857,980, filed Aug. 17, 2010, entitled “SYSTEM AND METHOD FORCONTROLLING A SOLAR PANEL OUTPUT”, issued as U.S. Pat. No. 8,791,602,which claims benefit of priority to Provisional U.S. Patent ApplicationNo. 61/234,540, filed Aug. 17, 2009, entitled “TEMPERATURE COMPENSATIONTO MAXIMIZE ENERGY HARVESTING FROM SOLAR PANELS AND TO REDUCE PANELAGING TO EXTEND USEFUL LIFE OF SOLAR PANELS”; the aforementionedpriority applications being hereby incorporated by reference in theirentirety.

TECHNICAL FIELD

The disclosed embodiments relate to a system and method for controllinga solar panel output.

BACKGROUND

A solar panel's operating point (voltage and current) is decided by anelectronic circuit called a maximum power point tracker (“MPPT”). As thetemperature increases, the MPPT drifts to produce a lower energy output.The V_(OC), or open circuit voltage, reduces significantly and I_(SC),or short circuit current, increases marginally.

FIG. 5 is a circuit equivalent of a conventional solar cell. R_(SH)represents the junction resistance of the cell.

Currently, MPPT based solar systems do not provide for temperaturecompensation. Thus, when panel temperature increases, the V_(OC) drops,and the panel works at new V_(MPP) and I_(MPP) values. There is nocorrection provided for reducing panel stress.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a control module for use in controlling an output ofa solar panel, according to an embodiment.

FIG. 2 illustrates the output of a solar panel in view of temperaturechanges.

FIG. 3 illustrates a control system for controlling an output of a solarpanel, according to embodiments.

FIG. 4 illustrates a process for controlling an output of a solar panel,according to embodiments described herein.

FIG. 5 is a circuit equivalent of a conventional solar cell. R_(SH)represents the junction resistance of the cell.

DETAILED DESCRIPTION

According to embodiments, a control system or module is provided toprovide temperature compensation that offsets the impact of increasedtemperature when a solar panel is in operation. Embodiments such asdescribed enable output from the solar panel to be maximized, whileenabling beneficial reduction in the temperature of the solar panel.Among other benefits, the longevity of the solar panel can be increasedby decreasing the temperature of the solar panel when it is operating athigh temperatures.

More specifically, embodiments recognize that when the temperature of asolar cell increases, it causes an increase in the junction resistance.This is because mobility of the charge carriers is inverselyproportional to temperature. As the temperature rises, there isincreased carrier scattering on lattice vibrations (phonons) andimpurities. This decrease in mobility causes a decrease in theconductivity and an increase in the series resistance.

Embodiments recognize that an increase in the temperature of a solarcell also results in a slight increase in the short circuit current. Theshort circuit current from a solar cell is the photocurrent. As thetemperature increases, the bandgap decreases, and as a result, photonsof a lower wavelength (lower energy) are absorbed by the solar cell,which results in a slight increase in photocurrent. It is believed thatdue to the increase in temperature, series resistance increases, whichimpacts the current downwards. However, due to increase in temperature,photons with lower wavelengths are absorbed by the solar cell whichcauses the current to go higher. As a net effect, current increasesmarginally with an increase in temperature. Furthermore, the bandgapdecreases with increasing temperature, more electrons are able to moveinto the conduction band. The extra electrons in the conduction band andthe holes in the valence band lead to an increase in the dark current.An increase in the dark current results in a decrease in the opencircuit voltage.

Furthermore, embodiments described herein recognize that heatdissipation is a contributor to panel aging. As such, embodiments asdescribed can be implemented to lower heat dissipation, and thusincrease panel life at a relatively lower power output.

In particular, the operating temperature of the solar panels can belowered without impacting energy harvesting from solar panels, systems,modules, cells or devices. A slight reduction in current helps reducethe power dissipation, and thus helps lower the panel temperature. Sucha slight reduction in current will increase the voltage and result inthe power output to be almost the same.

Embodiments described herein include a control system for a solar panel.The control system includes a control module and one or more inputsources. The control module is coupled to an output of the solar modulein order to operate the solar panel so that an output of the solar panelis at a maximum power level. The control module is able to selectivelydecrease a current level of the solar panel's output in response to acondition that is indicative of a temperature of the solar panel whilemaintaining the power output of the solar panel at or within adesignated percentage of the maximum level. The input source is coupledto the control module to provide an input that is indicative of thetemperature.

According to some embodiments, by reducing currents drawn from a module,cell or device, the open circuit voltage (“V_(OC)”) can be increased toa higher voltage. As V_(OC) increases, the maximum power point (“MPP”)voltage also increases, and thus, leads to a decrease in power loss.Furthermore, in some embodiments, temperature compensation can reducethe current, which is expected to reduce the temperature slightly (dueto the lower loss in R_(S) (series resistance of the solar cells) andR_(SH) (shunt resistance of the solar cells)).

Still further, embodiments described herein can generate maximum energyfrom solar panels at significantly reduced stress levels. Reducing thestress levels of the solar panels can help increase the operating lifeof solar panels.

FIG. 1 illustrates a control module for use in controlling an output ofa solar panel, according to an embodiment. In FIG. 1, a control module100 is shown for modifying an output of a solar panel so as to reducecurrent output and the operating temperature of the solar panel. Thecontrol module 100 includes temperature detection logic 110, powerdetection logic 120, and output modification logic 130. The controlmodule 100 controls the output of a solar panel by affecting currentlevels on the solar panels output. More specifically, as described withFIG. 2 and elsewhere, the control module 100 is operable to reducecurrent levels on the solar panel's output (so as to reduce temperaturelevels of the solar panel), while maintaining a power level of the solarpanel at or near a maximum level.

The temperature detection logic 110 uses temperature input 112 from thesolar panel (see FIG. 3) to determine the temperature level of the solarpanel. The power detection logic 120 uses sensors (see FIG. 3) on theoutput lines of the solar panel to determine voltage and current levels.

The output modification logic 130 is coupled to hardware or otherresources for affecting current levels on the output of the solar panel.As described with one or more other embodiments, the controlmodification logic 130 can affect a switching element of the solar paneloutput, in order to increase or decrease the current levels of the solarpanel output.

Accordingly, the output modification logic 130 provides a modificationoutput 132 that controls the hardware or resource for reducing thecurrent level of the solar panel output.

FIG. 2 illustrates the output of a solar panel in view of temperaturechanges. The output of the solar panel may be described in terms of (i)an open circuit voltage (V_(OC)), and (ii) a short circuit current(I_(SC)). The product of the open circuit voltage and the short circuitcurrent is the power level of the solar panel's output. The maximumpower point (“MPP”), or maximum power level, occurs at about the pointof inflection in each of the power graphs shown.

As shown by FIG. 2, the output of the solar panel can change as a resultof temperature increase in the solar panel. In particular, the current(I_(SC)) increases slightly if there is an increase in temperature, andthe voltage V_(OC) drops significantly. Embodiments recognize that asthe temperature changes, the MPP moves inwards or outwards depending onthe respective increase or decrease in temperature. In general, MPPmoves inward when the temperature increases (see graph line A at hightemperature and graph line B at less). This is due to the fact that withthe increase in temperature, Voc decreases while I_(SC) stays relativelyunchanged (or can increases slightly).

There are several methods to track the MPP. These methods are known asMPP tracking (or MPPT). In one embodiment, the temperature is monitoredusing temperature sensors such as a thermistor, using semiconductors orusing any other method.

With reference to FIG. 2, control module 100 can be implemented to forcecurrent reduction (i.e. reduce I_(MPP)) in the output of the solarpanel. The reduction in current, when appropriately measured, results inan increase of the solar panel's output voltage (i.e. increase V_(MPP)).Additionally, the reduction in current reduces the temperature of thepanel, thus enhancing longevity of the solar panel. According to someembodiments, the forced reduction in current may be implemented alongwith MPPT, so that the forced current reduction is iterative andoptimized to sustain power levels at or near maximum.

FIG. 3 illustrates a control system for controlling an output of a solarpanel, according to embodiments. A system 200 includes a control modulesuch as described with an embodiment of FIG. 1. Accordingly, system 200is coupled to affect current levels on an output line 202 of a solarpanel 260. The system 200 includes control module 100, output controlelement 210, and one or more detectors 220 for detecting voltage andcurrent on the output lines of the solar panel. Additionally, system 200may include a temperature sensor 222 which detects a temperature of thesolar panel during its operation.

The control module 100 may be implemented by a processor or integratedcircuit device. The control module 100 receives a temperature input 221from the sensor 222. In addition, the control module 100 receives inputthat indicates the voltage and current levels of the output line 202.The output modification logic 130 (see FIG. 1) of the control module 100implements a process to use the inputs from the temperature sensor 222and the voltage/current detectors 220. The algorithm implemented by thecontrol module 100 results in the output control 210 reducing thecurrent level on the output line 202. In one embodiment, the algorithmof the control module 100 implements MPPT with adjustment to currentlevels based on temperature input.

According to some embodiments, the output control 210 is a switchingelement. In particular, one a more embodiments provide that the outputcontrol 210 is a buck-boost switching element, capable of bucking orboosting the output voltage (i.e. Voc) on the output line 202. In oneimplementation, the switching element is formed by a combination ofMOSFETs or other transistors. The gate of the MOSFETs is controlled bypulse width modulation from the control module 100. The control module100 reduces the current level on the output line 202 by changing theswitching speed of the switching element. In this regard, the controlmodule 100 may signal a pulse width modulus (PWM) control signal toaffect the operation of the switching elements in reducing the currentlevels (or conversely, increasing the current levels) based on therequirements of the algorithm and/or other conditions.

FIG. 4 illustrates a process for controlling an output of a solar panel,according to embodiments described herein. A method such as described byFIG. 4 can be implemented using a system of FIG. 3. Accordingly,reference is made to elements of FIG. 3 for purpose of illustratingsuitable components or elements for performing a step or sub-step beingdescribed.

According to an embodiment, the determination as to whether currentreduction (and temperature reduction) is to take place is predicated ontriggering conditions being present. In an embodiment, the triggeringconditions include (i) temperature precondition, and (ii) power outputprecondition. In one embodiment, the temperature of the solar panel isidentified (310) in order to determine whether the solar paneltemperature is above a threshold temperature (315). In general, thetemperature of the solar panel can increase or decrease due to two mainfactors: (1) environmental factors decided by nature, and (2) heatproduced due to the current through the series (R_(S)) and shunt(R_(SH)) resistors of the panel and cables. If the temperature is notabove the threshold, the current reduction is not implemented, butfurther temperature monitoring may take place (318) until thetemperature condition (e.g. temp>threshold) is present.

Current reduction modification may be implemented as a means to reducetemperature on the solar panel. As such, such modification may beavoided when the temperature of the solar panel is less than a thresholdtemperature. In such cases, the temperature of the solar panel may berepeatedly monitored (318) to determine if the condition occurs in whichthe temperature of the solar panel is above the temperature threshold.

If the temperature condition is present (e.g. the solar paneltemperature is above the threshold), then an embodiment provides fordetermining the power level of the output 202 from the solar panel(320). As such, the current and voltage of the output 202 is detected. Adetermination is made as to whether the output of the solar panel is at(or within a designated percentage of) a maximum power level (325). Themaximum power level may be determined from logic or data residing withthe control module 100. An MPPT algorithm may be performed, for example,as part of steps 320 and 325 to achieve MPP. In one embodiment, thecontrol module 100 uses pulse width modulation to control switchingparameters of a switching element (e.g. MOSFET) used to control voltageand current on the output line 202 of the solar panel. The PWMparameters can be selected by an MPPT algorithm based on the inputparameters (such as panel current and panel voltage provided by thesensors shown in FIG. 3). The sensing is performed at a fixed intervaldetermined by the PWM frequency. The product of sensed voltage andcurrent, which is instant power, is compared to a previous value. If thenew instant power is higher than the previous value, the PWM width isincreased, thus increasing the current value. This process is continueduntil the new instant value of power is lower than previous value. Thispoint is known as the MPP and the PWM width is fixed at this level.

According to embodiments, current reduction modification of the solarpanel output is possible only when the power level of the output is ator within a designated percentage of the maximum power level. Thedesignated or threshold percentage of the maximum power level may be setin part by design parameters. For example, current reductionmodification may take place when the power level of the output is at orwithin 95% (or alternatively 99%) of the maximum power level. Otheracceptable ranges (e.g. 90%) may also be used.

If triggering condition are present corresponding to (i) the power levelof the solar panel being at a maximum (i.e. operating at MPP), and (ii)the temperature of the solar panel being above a threshold, then thecontrol module 100 implements current reduction control (330). In oneembodiment, control module 100 changes the switching speed of thecontrol element 210 coupled to the output 202 of the solar panel inorder to reduce current levels of the output. Still further, atemperature compensation circuit may be used to force the current to belower than its MPP value by controlling the PWM. As a result, lowercurrent runs through the series (R_(S)) and shunt (R_(SH)) resistors,thereby producing less power dissipation. The reduction in current isaccompanied by a corresponding increase in voltage, so that almost thesame amount of power is supplied from the solar panel. This new MPP istemperature compensated using the approach described above. In typicalsituations, the new MPP will be higher than the non-compensated V_(MPP)and lower than non-compensated current at MPP (I_(MPP)).

Once current reduction takes place, one embodiment provides that thecontrol module 100 determines the temperature of the solar panel usinginput from the temperature sensor 222 (340). The temperature condition,or new condition, may be checked as part of the solar panel's responseto the current reduction (345). The second temperature condition maycorrespond to, for example, (i) a determination of the temperature onthe solar panel as a result of the current reduction, or (ii) adetermination of the change in temperature as a result of the currentreduction. For example, if the difference between the currenttemperature of the solar panel and its previous measurement is greaterthan some threshold, then the current reduction may be continued (orperformed again). However, in many instances, temperature reductionceases after one or more iterations of current reduction. At such point,some embodiments may cease current reduction (360).

The voltage and current can be detected again (350) to determine whetherthe power level of the solar panels output is at or within thedesignated percentage of the maximum level. If the power level hasdropped, current reduction may be stopped (360) or even reversed.However, if the power level has not significantly dropped (e.g. so thatit is within the designated percentage of the maximum), then currentreduction may be performed again (assuming temperature condition 345remains true).

While embodiments such as described with FIG. 3 and FIG. 4 usetemperature input as a trigger, other embodiments may incorporate othertriggers or input to initiate current and temperature reduction. Forexample, with reference to FIG. 3, control module 100 may receive inputfrom a clock 205. The clock 205 may indicate temperature by correlationwith time of day. For example, as an alternative to temperaturedetection, clock 205 may trigger the control module 100 to implementcurrent reduction during a particular period of the day. Optionally,information from the clock 205 can be cross-referenced with time of yearand/or geographic location.

As another variation, other sources of information may include forexample of a data port (not shown) which communicates environmentaltemperature to the control module 100.

Still further, the control module 100 may be operated under the remoteand programmatic command of a larger system, such as provided by aservice. In such embodiments, the control module 100 may include a dataport for receiving communications from a remote service.

The following provides a simplified example of the operation of acontrol system or module, in accordance with embodiments describedherein:

-   -   Operating Temperature: 25 degrees C.    -   For a 80 W PV panel, typically, V_(OC)=22 V, I_(SC)=7.6 Amps.        The MPP at room temperature is typically is V_(MPP)=17 V        (Voltage at MPP), I_(MPP)=6.0 A (Current at MPP), Power at MPP:        17*6=102 W.    -   At panel temperature Temp=55 degrees C., and without temperature        compensation, the expected measurements are V_(OC)=19.4 V,        V_(MPP)=13.4 V, I_(SC)=7.6 Amp and I_(MPP)=6.01 Amp.    -   With temperature compensation (e.g. by current reduction), at        Temp=55 degrees C., by forcing I_(MPP) (Current at MPP) to 5.0        Amp, V_(MPP) (Voltage at MPP) will boost to 15.6 V to produce        the same amount of power. As an example, due to reduction in        current, power lost in R_(SH) is reduced, thus reducing        temperature by up to 12 degrees C. depending on the thermal        resistance of the mounting.

CONCLUSION

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments. As such, many modifications and variations will be apparentto practitioners skilled in this art. Accordingly, it is intended thatthe scope of the invention be defined by the following claims and theirequivalents. Furthermore, it is contemplated that a particular featuredescribed either individually or as part of an embodiment can becombined with other individually described features, or parts of otherembodiments, even if the other features and embodiments make nomentioned of the particular feature. This, the absence of describingcombinations should not preclude the inventor from claiming rights tosuch combinations.

What is claimed is:
 1. A method for controlling an output of a solarpanel, the method comprising: determining a temperature input for thesolar panel; and when the temperature input is above a threshold,selectively reducing a current output of the solar panel whilemaintaining a power level of the output of the solar panel above adesignated threshold.
 2. The method of claim 1, wherein selectivelyreducing the current output of the solar panel includes: when thetemperature input of the solar panel is above the threshold, determiningwhether the power level of the output of the solar panel is within adesignated range of a maximum power level, and reducing the currentoutput of the solar panel when the power level of the output of thesolar panel is within the designated range.
 3. The method of claim 2,wherein determining whether the power level of the output of the solarpanel is within the designated range of the maximum power level includesdetermining a maximum power point of the output of the solar panel. 4.The method of claim 1, wherein determining the temperature inputincludes determining an input from a temperature sensor that is coupledto the solar panel.
 5. The method of claim 1, wherein determining thetemperature input includes determining a time of day from a clock input.6. The method of claim 1, wherein determining the temperature inputincludes determining a data input provided through a data port.
 7. Themethod of claim 1, wherein further comprising: after reducing thecurrent output of the solar panel, making a determination of whether atemperature condition exists for the solar panel.
 8. The method of claim7, further comprising reducing the current output again when thetemperature condition is determined to exist.
 9. The method of claim 8,wherein the temperature condition includes determining, after reducingthe current output of solar panel, that a temperature of the solar panelis within a designated range of a temperature of the solar panel beforethe current output of the solar panel is reduced.
 10. A system forcontrolling an output of a solar panel, the method comprising: atemperature input; and a control module coupled to the temperature inputand to the output of the solar panel to: determine a temperature inputfor the solar panel from the temperature input; and when the temperatureinput is above a threshold, selectively reduce a current output of thesolar panel while maintaining a power level of the output of the solarpanel above a designated threshold.
 11. The system of claim 10, whereinthe control module selectively reduces the current output of the solarpanel by (i) determining, when the temperature input of the solar panelis above the threshold, whether the power level of the output of thesolar panel is within a designated range of a maximum power level, andthen (ii) reducing the current output of the solar panel when the powerlevel of the output of the solar panel is within the designated range.12. The system of claim 11, wherein the control module determineswhether the power level of the output of the solar panel is within thedesignated range of the maximum power level by determining a maximumpower point of the output of the solar panel.
 13. The system of claim10, wherein the temperature input includes a temperature sensor that iscoupled to the solar panel.
 14. The system of claim 10, wherein thetemperature input includes a clock input.
 15. The system of claim 10,wherein the temperature input includes a data port.
 16. The system ofclaim 10, wherein, after reducing the current output of the solar panel,the control module makes a determination of whether a temperaturecondition exists for the solar panel.
 17. The system of claim 16,wherein the control module reduces the current output again when thetemperature condition is determined to exist.
 18. The system of claim17, wherein the control module determines that the temperature conditionexists by determining, after reducing the current output of solar panel,that a temperature of the solar panel is within a designated range of atemperature of the solar panel before the current output of the solarpanel was reduced.
 19. The system of claim 10, wherein the controlmodule reduces the current level by controlling one or more switchesthat regulate the output of solar panel.
 20. A system for controlling anoutput of a solar panel, the system comprising: means for determining atemperature input for the solar panel; and means for selectivelyreducing a current output of the solar panel while maintaining a powerlevel of the output of the solar panel above a designated threshold whenthe temperature input is above a threshold.